Chemical mechanical polishing (CMP) in the semiconductor industry is typically used to planarize the surface of a dielectric film or to remove excess metal film deposited on a substrate or previous film. To perform planarization, a relatively hard pad material is used such as polyurethane with special filler material. To provide global (within wafer) planarization, a softer under layer is generally used. The effectiveness of a planarization process strongly depends on the hardness of the polishing pad which removes the high spots of a dielectric film in the presence of a polishing slurry. Therefore, since the planarization is a primary objective for performing CMP, hard pads are generally used. This planarization step is referred to as the primary or first polishing step.
Unfortunately, a hard polishing pad is not favorable for achieving low defects on the polished surface. Therefore, a second platen (polishing table) is used with a softer polishing pad either to polish off small amount of material after the primary polishing step or to simply "clean" the surface of the wafer prior to the cleaning system. This "cleaning" step on the separate polishing table uses DI water and is often called buffing. The separate polishing table on the polishing machine adds complexity, reduces throughput and increases the footprint of the polishing tool. It is desirable to perform all wafer cleaning operations outside the polisher to avoid these disadvantages.
Regular wafer scrubbing systems can successfully remove loose slurry particles from the surface of the wafer and some scrubbing systems can remove metallic contamination from the wafer surface. Regular scrubbers are not effective in removing small slurry particles that are often embedded into the film. Normally buffing on the soft polishing pad removes the majority of such defects leaving the polished surface sufficiently clean and prepared for scrubbing.
It is therefore desirable to enhance scrubber performance and clean wafers after the primary CMP step to provide the same defect level as the two step CMIP process, specifically: primary polishing step plus the buffing step.
The prior art wafer scrubbing systems have not been adapted for buffing operation because the pressure and speed of the buffing media was significantly lower than it is required for buffing. Some of the scrubbing systems do not lend themselves to perform buffing because the wafer is being held by the edge and inducing the typical for buffing pressure may break the wafer.
The present invention provides an improved wafer cleaning system specifically designed to perform a buffing operation. The system allows wafers to be processed with a scrubber with significantly increased pressures and speeds broadening the range of applications. The apparatus also allows control and change of processing parameters to perform buffing or regular scrubbing in the same or separate stations when applying suitable chemicals.